Control of the radial clearance between the tips of rotating blades and the surrounding annular shroud in axial flow gas turbine engines improves engine efficiency. For example, by reducing the blade tip to shroud clearance, designers can reduce the quantity of turbine working fluid which bypasses the blades, thereby increasing engine power output for a given fuel or other engine input. On the other hand, blade tip to shroud contact leads to friction losses and wearing of parts. “Active clearance control” refers to clearance control arrangements wherein a quantity of working fluid, such as air, is employed by the clearance control system to regulate the thermal expansion of engine structures, thereby controlling the blade tip to shroud clearance.